C-Glossary
RAID 3 utilizes a striped set of three or more disks with the parity of the strips (or chunks) comprising each stripe written to a disk. Note that parity is not required to be written to the same disk. Furthermore, RAID 3 requires data to be distributed across all disks in the array in bit or byte-sized chunks. Assuming that a RAID 3 array has N drives, this ensures that when data is read, the sum of the data-bandwidth of N – 1 drives is realized. The figure below illustrates an example of a RAID 3 array comprised of three disks. Disks A, B and C comprise the striped set with the strips on disk C dedicated to storing the parity for the strips of the corresponding stripe. For instance, the strip on disk C marked as P(1A,1B) contains the parity for the strips 1A and 1B. Similarly the strip on disk C marked as P(2A,2B) contains the parity for the strips 2A and 2B. Advantage: RAID 3 ensures that if one of the disks in the striped set (other than the parity disk) fails, its contents can be recalculated using the information on the parity disk and the remaining functioning disks. Consequently read operations can be time-consuming when the array is operating in degraded mode. If the parity disk itself fails, then the RAID array is not affected in terms of I/O throughput but it no longer has protection from additional disk failures. Also, a RAID 3 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set. Disadvantage: Due to the restriction of having to write to all disks, the amount of actual disk space consumed is always a multiple of the disks’ block size times the number of disks in the array. This can lead to wastage of space.
RAID 5 A RAID 5 array is similar to a RAID 4 array in that, it utilizes a striped set of three or more disks with parity of the strips (or chunks) comprising a stripe being assigned to the disks in the set in a round robin fashion. The figure below illustrates an example of a RAID 5 array comprised of three disks – disks A, B and C. For instance, the strip on disk C marked as P(1A,1B) contains the parity for the strips 1A and 1B. Similarly the strip on disk A marked as P(2B,2C) contains the parity for the strips 2B and 2C. Advantage: RAID 5 ensures that if one of the disks in the striped set fails, its contents can be extracted using the information on the remaining functioning disks. It has a distinct advantage over RAID 4 when writing since (unlike RAID 4 where the parity data is written to a single drive) the parity data is distributed across all drives. Also, a RAID 5 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.
RAID 10 A RAID 10 array is formed using a two-layer hierarchy of RAID types. At the lowest level of the hierarchy are a set of RAID 1 arrays i.e., mirrored sets. These RAID 1 arrays in turn are then striped to form a RAID 0 array at the upper level of the hierarchy. The collective result is a RAID 10 array. The figure below demonstrates a RAID 10 comprised of two RAID 1 arrays at the lower level of the hierarchy – arrays A and B. These two arrays in turn are striped using 4 stripes (comprised of the strips 1A, 1B, 2A, 2B etc.) to form a RAID 0 at the upper level of the hierarchy. The result is a RAID 10. Advantage: RAID 10 ensures that if one of the disks in any parity group fails, its contents can be extracted using the information on the remaining functioning disks in its parity group. Thus it offers better data redundancy than the simple RAID types such as RAID 1, 3, and 5. Also, a RAID 10 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.
